Field probes are used in evaluating the performance of many different kinds of radio frequency equipment, and are also essential components of radiated immunity testing systems for use in ensuring that electrical and electronic and systems in products such as automobiles are not adversely affected by stray fields such as radio and television transmissions, radar pulses, cellular telephone signals, powerline fields, and the other kinds of electromagnetic fields.
Several different kinds of field probes are used in the above-mentioned applications. These include cube probes, sphere probes, and so-called “stalk probes.” A typical cube probe includes three mutually perpendicular sensing elements extending from three faces of a cube-shaped housing containing electronic circuitry. A sphere probe is similar to the cube probe, but the spherical shape of the electronics housing helps to prevent probe orientation from affecting field measurements. In the stalk probes, sensors are disposed in an RF-transparent housing and connected to associated electronic circuitry by a transmission line extending through an elongated tube, thereby keeping the electronic circuitry remote from the sensors in order to minimize interference by the electronic elements with the incident radiation.
A typical probe sensor includes a detecting diode that delivers a DC signal the amplitude of which corresponds to the magnitude to the field incident on the sensor. The electronic circuitry in these probes processes the DC signals, delivering a composite signal representing the “X”, “Y” and “Z” axis field components as the square root of the sum of the squares of the individual components. The circuitry can also supply signals representing the X, Y and Z components independently. The circuitry typically also extends the dynamic range of the probe, improves the linearity of the probe's response, controls sampling, and protects against overload.
With each of the three types of probes, it is desirable to avoid the use of conductive wiring to supply electric power and control signals to the electronic circuitry and to deliver data from the electronic circuitry to external monitoring equipment. Accordingly, it has been common to utilize electrochemical cells to supply power to the probes' internal electronic circuitry and to use optical fibers to deliver data to the monitoring equipment. It is also common to utilize laser radiation to supply operating power to a probe's electronic circuitry, supplying laser light through an optical fiber to a converter that generates DC operating power for the electronic circuitry.
Broadband field probes, utilizing short dipole antennas formed by thin film resistive strips with diodes in series with the strips to convert RF to DC signals, have been described by Samuel Hopfer in U.S. Pat. No. 4,207,518, granted Jun. 10, 1980 and U.S. Pat. No. 4,392,108, granted Jul. 5, 1983. These patents have formed the basis for a number of practical electric field probes having a high bandwidth, and the entire disclosures of both patents are herein incorporated by reference.
Problems encountered with conventional broadband field probes of the kind mentioned above include difficulties in measuring DC voltage across a detector diode situated at the mid-point of a short dipole, avoidance of metal objects near the sensors, and avoidance of conductors for coupling the sensors to the electronic circuitry. A resistive connection between the sensors and the electronic circuitry has generally been considered to be the best solution to the problems posed by metal objects and conventional conductors, and carbon feed lines have been typically used.
The connections between the feed lines and the sensing elements give rise to manufacturing difficulties. It is difficult to make mechanically reliable connections that do not impair the probe's frequency response.